Signal-translating system and method of operation



Jan. 13, W42. H. M. LEWIS ET AL SIGNAL-TRANSLATING SYSTEM AND METHOD OF OPERATION Filed Aug. 2, 1959 3 Shee'tsSheet l -|NVENTOR HAROLD M. LEWIS mm: 0. WILSON MM ATTORNEY Jam 13, 1942. H. M. LEWIS ET AL 2,269,590

SIGNALTRANSLATING SYSTEM AND METHOD OF OPERATION Filed Aug. 2, 1939 3 Sheets-Sheet 2 FlG.8.

MODULATOR OSCILLATOR INVENTOR HAROLD M. LEWIS JOHN 0. WILSON ATTORN EY ETHOD OF OPERATION 1939 3 Sheets-Sheet 5 Jan. 13, 1942. H. M. LEWIS ET AL SIGNAL-TRANSLATING SYSTEM AND M Filed Aug 2 law curve.

Patented Jan. 13, 1942 SIGNAL-TRANSLATING SYSTEM AND METHOD or cram-non Harold M. Lewis, Great Neck, and John C. Wilson, Bayslde, Long Island, N. Y., assignors to Hazel tine Corporation,a corporation of Delaware Application August 2, 19:9, SeriaLNo. 287,912 Claims. (01. its-4.1

'This invention relates to a television signaltranslating stage for translating a television signal having a predetermined amplitude level corresponding to a desired condition for reproduction and including a repeater stage having a signal input-signal output characteristic which follows substantially a power-law curve so that the signal is distorted in accordance with the law of the characteristic and also relates to methods of translating a televisionv signal so that the distortion of translation follows a powering the gamma of a video-frequency television signal.

In photography, it is found that if pictures are developed with a linear relationship between the shade values of the picture and those of the object, the picture is likely to ,be dull and uninteresting. It has, therefore, been the practice to introduce a predetermined distortion in the developing of pictures in order to overcome this difliculty. A power-law distortion is customarily used and a picture so corrected is said to have a gamma equal to the power or exponent of the curve which the distortion follows. If a picture which has a linear relationship between the shade values of the picture and the actual corresponding values of the object isgiven -a gamma of 2 and/isagain reproduced with a gamma of the result is a picture with a linear relationship between the shade values of the picture and the actual corresponding values of the object; that is, the picture now has a gamma of 1. Motion pictures are commonly processed to have a gamma of about 1.8. Inasmuch as motion pictures are frequently transmitted in television practice, it .is desirable to be able to provide a television system in which the true gamma of the television signal can be changed While the invention is of general 'application, it is of particular utility in modifyas desired in various stages of the system. Also,

in other types of television practice it is desirable to be able to provide and maintain a predetermined gamma of the transmitted picture.

Certain prior art television systems, while providing arrangements for producing a distortion of the translated signal with respect to the shade values thereof and while sometimes the distortion is in accordance with a power-law characteristic, havenot been designed with the concept of'properly relating the translated signal to the distorting power-law characteristic in order that the resultant distortion follow the power law of the characteristic. That is, a signal may be distorted by translation through a device having a signal input-signal output characteristic following a power-law curve, but the gamma of the signal is not adjusted in accordance with the true power law of the characteristic unless the signalbears a critical and predetermined relationship with respect to the characteristic. Thus, if'such a signal as translated is given some arbitrary distortion the gamma value of which varies from regionto region ofthe amplitude range, it is very difflcult again to apply the-signal to another distorting translating device in order .to obtain thereby a signal which has a linear reproduction or which has any other predetermined value of gamma. It is, therefore, apparent that it is desirable properly to relate the signal to a given translating characteristic in order properly to control the gamma of the translated signal.

Furthermore, if a television signal is stabilized on a particular shade value thereof, for instance, the value of picture black in the transmitted picture, and applied with the proper signal level at the axis'of the power-law curve signal-translating characteristic to provide a given gamma equal to the exponent of the curve, the signal will not be properly located with respect to the translating characteristic for different amplitude values of the same translated signal since the stabilization level, picture black, varies with such amplitude variations fromabsolute black which is generally at a considerably lower brightness level. The term amplitude. of the signal, as used in this specification, is' intended to denote the value of the signal dependent upon carrier strength, fading, and like effects, and is, therefore, independent of light-modulation components of the signal. It is, therefore, highly desirable to provide an arrangement for providing a predetermined gamma correction of translated television signals which is independent of varying amplitudes of the translated signal.

It is, therefore, an object of the invention to provide' an improved television signal-translating method in which one or more of the abovementioned disadvantages of the prior art are eliminated.

It is-a further object of the invention to provide a television signal-repeating method for modifying the gamma of a translated signal closely in accordance with the exponent of a power-law signal input-signal output translating characteristics It is another object of the invention to proistic together with means for providing a predetermined relation between the signal as applied to the stage and the characteristic the stage and for adjusting this relationship in accordance with the amplitude of the signal in it such manner that translated signals of all amplitudes receive the gamma. of the characteristic.

In accordance with a feature of the invention,

the method of imparting to a television signaltranslating system a translating characteristic of a predetermined gamma comprises, translating with a signal input-signal output distortion corcorresponding to a power-law curve a television signal including components representative of the. unidirectional background ofthe translated which are displaced from the axis of symmetry in accordance with their amplitude displacement from the related amplitude level. The method of the invention also comprises deriving an output signal corresponding to the distorted translated signal. 8

Also in accordance with a feature of the invention, a television signaL-translating system comprises. a s gnal-translating stage having a signal input-signal output characteristic approximating a power-law curve, means for applying 3 to the input circuit of the stage a video-frequency television signal of variable amplitude and including components representative of the unidirectional background of the translated picture,

and means for biasing the stage so that for a given amplitude of the signal a predetermined signal level related to the signal and outside the range of the signal-amplitude levels to be reproduced corresponds with the intersection of the power-law curve and an axis of symmetry there- 40 of. The system also comprises means for adjusting the bias of the stage in accordance with the amplitude of the translated signal, so that predetermined shade levels of the signal are maintained at points on the power-law characteristic which are displaced from the axis of symmetry in accordance with their amplitude displacement from the related signal level, as well as means for deriving an output signal from the stage.

For a better understanding of the invention, 5 together with other and further objects thereof, reference is had to the following description-- taken in connection with the accompanying drawings and its scope will be pointed out in the appended claims. 6

In the accompanying drawings, Fig. l is a schematic diagram of a complete television transmitting system of the scanning-disc type adapted to be operated in accordance with the present invention; Fig. 2 is an illustration of a 6 portion of the scanning disc of the arrangement of Fig. 1; Figs. 3 and 4 are graphs illustrating certain of the operatingcharacteristics of the transmitter of Fig. 1; Fig. 5 is a circuit diagram of a modification of the gamma adjusting stage 7 of the circuit of Fig. 1; Fig. 6 is a graph utilized to explain certain of the operating characteristics of the circuit of Fig. 5; Fig. 7 is a schematic circuit diagram of a complete television transmitting system of the cathode-ray type adapted to be operated in accordance with the present invention; while Fig. 8 is a schematic circuit diagram of a complete television receiving system embodying the present invention.

Referring now more particularly to Fig. l of the drawings, there is shown a schematic circuit diagram of a television transmitting system comprising a television signal generator I I of the scanning-disc type. Connected in cascade to the output circuit of signal generator I, in the order named, are video-frequency amplifiers ll, l2, and Illa limiter H, a video-frequency combining amplifier IS, a modulator I. having associated therewith a carrier-frequency oscillator I1, and a radiating system II, It, the schematic portions of the figure all being in accordance with conventional-television practice.

Synchronizing signals developed in signal generator II are applied through synchronizingpulse amplifier 2! to video-frequency combining amplifier It in a conventional .manner. Signal generator ll includes a scanning disc 25, a segment of which is illustrated in Fig. 2 of the drawings. The scanning disc 25 comprises two series of apertures 26 and 21 for deriving video-signal components and synchronizing-signal components, respectively, in a conventional manner. The'scanning disc 25 is rotated by means of a motor 28. Light from the object 28 to be televised is focused by means or lens system 3|, 3| through the apertures 26 and an aperture 22 in a stationary mask 32 upon photocell 34 in order to generate video-frequency signals. Light from a lamp II, focused by means of lenses 3! and 38 through the holes 21 of scanning disc 25 upon a photocell 29, is eflective to develop synchronizing signals for the transmitter system.

Neglecting for the moment the portions of the system of Fig. 1 relating to the present inven-' tion, the system just described comprises the elements of a television transmitting system of a conventional design and the various parts thereof may be of any well-known construction, rendering detailed description of the general system and its operation unnecessary. Briefly, however, the image 01' a scene to be transmitted is focused on the photocell 24 in the signal generator It and a video-frequency voltage is developed thereby in the usual manner and applied 'to video- 0 frequency amplifier Ii wherein this voltage is amplified and from which it is translated through video-frequency amplifiers l2 and ll to limiter l4 wherein the signal is limited so that only video-frequency portions of the signal are translated by succeeding stages of the system.' The output of limiter H is applied through videoirequency amplifier ii to modulator l6 wherein it is modulated with a high-frequency carrier signal derived from source I! and from which the modulated-carrier signal is translated to antenna system l8, it for radiation. Light from lamp I6 falling upon photoelectric unit 39 is effective to develop a synchronizing signal which is amplified in amplifier 20 and applied to the signal-translating channel of the receiver in video-frequency combining amplifier l 5 in a conventional manner.

Coming now to the portion of the system relating to the present invention, video-frequency amplifier l2 includes a vacuum-tube amplifier 40 to which the signal output from video-frequency amplifier I2 is applied through a coupling condenser 4i and direct current reinserter or stabilizer diode 42 across which is connected a load resistor 43. A suitableadiustable source of gridthis generated video signal may vary between the conventional amplifier tubes, it is apparent that a signal input thereto will be distorted to various bias voltage 44 is provided for vacuum tube 40. The output circuit of' vacuum tube 40 is conventional and includes; in series, a resistor 45, an inductance 48, and a sourceof unidirectional operatingpotential 41.

In considering the operation of the arrangement Just described, reference is made to Fig. 3

wherein the signal amplitude-object brightness characteristic of signal generator in is illustrated at curve A. It is assumed that the photocell 34 is operated between the limits of 28, representing the 'shade value of picture black which is generally of a shade value considerably brighter than absolute black, and W, representing the shade value of white in the transmitted picture. A signal may, therefore,'be assumed to be gene degrees depending upon the portion of the characteristic over which the signal-amplitude range extends. Assuming that it is desired to distort the signal with a true gamma of 2, it is apparent that-some particular amplitude value of the signal must correspond with some particular point on the characteristic of the curve 7:2. It has been found that, in order toamplify a signal such as that shown by curve C'of Fig. 3 with a gamma of 2, it is necessary so to relate the signal of curve C to the characteristic of =2 that the absolute erated having a wave form as represented by curve C. It is seen that the actual amplitude of signal-amplitude limits corresponding to B and I W on curve A and that, during the interval between successive scanning lines at which time none of holes 26 are in front of aperture 32 and all the light from object 29 is cut off from photocell 34, the generated signal amplitude is reduced to some low value corresponding to 'zero brightness level B0 on curve A, representing absolute black. The value of the signal during the portion of the cycle between successive scanning lines thus follows the form of the pulse a of curve C. It is apparent that the signal-amplitude value B0 is that corresponding to absolute black and that the signal from signal generator It may vary from this value B0 to the value W corresponding to white in the transmitted picture.

For the purposes of explanation it will be assumed that the signal C of Fig. 3 is generated with a linear relationship between the signalamplitude values thereof and the brightness values of the picture, this relationship being inherent due to the assumed straight-line characteristic A. As explained above, in some cases it may be very desirable to modify this derived signal in accordance with a power-law characteristic to change the gamma thereof in accordance with the power law of the translating characteristic. This may be done by means of vacuum tube 40 having an input voltage-output current characteristic which follows a power law. Several such tube characteristics in idealized form black level Bo thereof corresponds to an axis of symmetry of the power-law curve representing the input voltage-output current characteristic. Such a signal properly applied to the characteristic of -y=2 of Fig. 4 is illustrated by curve D of Fig. 4 wherein the darker shade values are more negative than the brighter shade values as applied to the,stage. nal-amplitude value B corresponding to picture black does not correspond to the cutoff point of the tube utilized and, therefore, prior art television systems, which have stabilized a videofrequency signal and applied it to a vacuum tube with the level corresponding to picture black stabilized at the cutoff point of the tube in order to adjust the gamma thereof, do not accurately provide the desired gamma correction.

. In Fig. 4, there is illustrated by curve E a signal corresponding to that of curve D but with twice the amplitude thereof. It is seen that the signal-amplitude value B corresponding to picthan at the level B, corresponding to picture black, as has been done in prior art practice. The signal output of the scanning-disc arrangement of Fig. Lis particularly suitable for such operation for the reason that, during the inter-.

vals between scanning lines, as shown by curve C of Fig. 3, the signal output of signal generator that each of these power-law curves has at least one axis of symmetry passing through the intersection of the input-voltage and output-current axes of Fig. 4. By the term "power-law curve, as used in this specification, is meant a curve having exponents other than unity. Thus, the curve of -y=2 is symmetrical about the outputcurrent axis while the curve'of =3 is symmetrical about both co-ordinate axes.

Assuming for the purposes of explanation that the tube 40 has the characteristic of 7:2 illustrated in Fig. 4, which is approximated in many I0 has a value Bo corresponding to absolute black.

Diode reinserter 42 serves to reinsert the unidirectional component of the signal in a conventional manner and bias source 44 may be adjusted to obtain the proper relation of the signal as applied to the characteristic of stage 40. fter the signal has been given the desired gamma by proper translation through stage i3. it may be limited at the picture black level in accordance with prior art practice by means of limiter l4 and translated by conventional television signaltranslating'stages which are effective to maintain the gamma thereafter or to provide some other gamma correction in accordance with the disa closure of the present invention.

While the curve of :2 has been chosen for the purposes of illustration, it will be understood that other values of gamma; corresponding to other power-law curves having exponents greater than unity, can be provided by choosing a tube having a proper characteristic and operating the tube in a manner similar to that described above.

In order to provide a gamma correction of less than unity, the circuit of Fig. 1 may be modified as illustrated in Fig. 5 to incorporate the stage of video-frequency amplification I3 in place of It is thus seen that the sigtube 4" having an input voltage-output current characteristic which is concave in the upward direction and the stabilizing diode 42' is coupled' with a polarity opposite that of diode 42 of videofrequency stage ii, that is, the darker shade values of the signal are less negative than the brighter shade values'thereof. This input voltage-output current characteristic may be provided by a properly selected tube of the remote cutoff type. The other portions of Fig. are identical to corresponding portions of Fig. 4 and have the. same reference numerals. In considering the operation of the circuit of Fig. 5, it will be amumed that the tube 40' has an input voltage-output current characteristic as illustrated by the curve of of Fig. 6. The curves of Fig. 6 are identical with those of Fig. 4 except that they have been rotated through 180 degrees.

It is thus seen that, in order to apply the signal go D' of Fig. 6 to the 'curve of with the absolute black level B0 thereof stabilized on an axis of symmetry of the power-law curve, it is necessary to provide a signal input in which the signal-amplitude corresponding to the shade white 2 is more negative than that corresponding to the shade black. The signal so stabilized may be applied to any of the other fractional exponent curves, for instance, to the curve of 'y='/; of

, manner. A timing-impulse generator 54 is pro Fig. 6, in the manner illustrated, to provide a so true gamma correction of the translated signal.

In summary, therefore, it is seen that the method of imparting to a television signal-translating system a translating characteristic of a predetermined gamma, which has just been described, comprises translating with a signal input-signal output distortion corresponding to a power-law curve, for example, corresponding to the characteristic v= /2, Fig. 4, a television signal as represented by curve D of Fig. 4 including components representative of the unidirectional background of the translated picture. The method also comprises effectively maintaining a predetermined amplitude of the received signal related to, and outside of, the amplitude range of signals to be reproduced, specifically, the predetermined amplitude level Bo related to the signal and representing the level of absolute black in the signal, at the intersection of the abovementicned characteristic and an axis of symmetry thereof. Accordingly, other predetermined amplitudes of the signal, corresponding to the amplitude range of signals to be reproduced, correspond to points on the curve 'y=2 which are displaced from the axis of symmetry in accordance with their amplitude displacement from the related level, or, specifically, in accordance with their displacement from the amplitude level corresponding to the shade B0 of absolute black. The method also comprises deriving an output signal corresponding to the distorted translated signal.

Referring now to Fig. '7, there is shown a schematic circuit diagram of a complete television transmitting system, adapted to be operated in accordance with the invention and utilizing a"- conventional cathode-ray tube signal generator ID. The system of Fig. 7 is generally similar to that of Fig. 1 and corresponding elements have the same reference numerals. Signal generator ll includes a conventional cathode-ray signal-generating tube 50 having a photomosaic target il upon which an image of an object 53 is focused by means of lens system 52 to generate video-frequency signals in a conventional 75 vided for synchronizing the various units of the transmitter. Field-frequency generator and line-frequency generator II are provided for defleeting the cathode-ray beam of tube 50 in a conventionalmanner. A pedestal-impulse generator i1 is provided having its output coupled to video-frequency amplifier ll. Synchronizing signals generated in unit II are inserted in the video-frequency channel at amplifier l8 as in the system of Fig. 1.

Inasmuch as no direct current component is developed by a cathode-ray tube signal generator, it is necessary to provide a separate unit for this purpose in signal generator I0. This unit includes a photocell it upon which light is concentrated from the object 53 by means of a lens ii. In order to combine the generated video-frequency signals including the pedestals inserted by pedestal-impulse generator 51 and the direct current components of the signal generated by phototube 6D and to provide limiting of the signal at the absolute black level Bo thereof, there is coupled between the output circuit of video-frequency amplifier II and videofrequency amplifier 13 a signal-translating stage 62 including a vacuum tube 63. The signal output of phototube is applied to the input elec. trodes of tube 63 through a direct current amplifier 64. A source of adjustable grid-bias voltage 65 is provided for tube 63, the output circuit thereof being conventional and including a series-connected resistor 66, inductance 61, and unidirectional power-supply source 68. Unit II of Fig. 7 is in all respects identical to unit ll of Fig. l and similar circuit elements have identical reference numerals.

In considering the operation of the system of Fig. 7 and neglecting for the moment the operation of the portion of the system relating to the instant invention presently to be described, it will be seen that video-frequency signals generated in signal generator III are amplified in video-frequency amplifier -II and have inserted therein pedestal impulses from generator 51. This -video-frequency signal is amplified in unit 62 and'is translated therefrom to unit IS, the operation of the remaining portions of the videofrequency signal-translating channel of the system being identical with that described for the system. of Fig. 1. Timing impulses generated in generator 54 control the synchronizing of the field-frequency generator 55 and line-frequency generator 56 to provide scanning voltages for generator 51 and the synchronizing-signal generator 58.

Coming now to the portion of the system relating to the present invention, it will be seen that a video-frequency signal generated by cathode-ray signal-generating tube 50 is centered about its alternating current axis in a manner which is well understood in the art. It will be assumed that there is mixed with this generated video signal blanking pulses of large amplitude in the negative direction during the retrace intervals and that the output of video-frequency amplifier ii is combined in amplifier G3 with the direct current component of the signal generated in phototube 00. There is, therefore, present in the input circuit of vacuum tube 83 a resultant signal which includes video signals which are stabilized at the level of picture black B and which include also very large negative plifier ll of Fig. l and illustrated by curve C of Fig. 3 and the operation of the remaining portion of the system is identical to that described with'reference to Fig. 1.

From an inspection of curves D and E of Fig. 4, it will be seen that the bias voltage effectively displacing the black level B of the picture from the level of absolute black Bo, located at 'an axis of symmetry with respect to the characteristic curves. should vary directly in accordance with the amplitude of the signal input to the system. Thus. it is possible to utilize the invention by applying a signal stabilized and limited at the level of picture black to the input circuit of a vacuum tube it a bias voltage of proper magnitude and polarity is provided which varies directly in accordance with the amplitude of the signal input to the system. An arrangement of this kind is illustrated in the receiver circuit of Fig. 8 to which attention is now directed.

The receiver circuit of Fig. 8 comprises a radiofrequency amplifier 10 having input terminals connected to an antenna circuit 1|, 12 and output terminals coupled to a frequency changer or oscillator-modulator 13. Connected in cascade with the output circuit of frequency changer 13, in the order named, are an intermediate-frequency amplifier 14, an intermediate-frequency amplifier 15, a signal detector 16, a video-frequency amplifier 11, a diode reinserter 18, and a cathode-ray picture-reproducing unit 19. Considering first the operation of the receiver as a tube 11 is conventional and includes a load re sister 84 and a unidirectional source 85. a coupling condenser 86 serving. to couple the videoi.'requency signal output 0! tube 11 to the cathode-ray signal reproducer 19.

In order to provide a bias voltage for the input circuit of tube 11 which increases directly in accordance with the amplitude of the received signal. there is provided a rectifier 81 coupled to the intermediate-frequency channel of the receiver by means 01' a secondary winding 88 on an intermediate-frequency coupling transformer 89, interposed between intermediate-frequency amplifiers 14 and 15. Suitable self-bias circuits are provided for tubes 14 and 15 comprising cathode resistors 8i by-passed for carrier-frequency signals by condensers 92. Unidirectional supply sources 93, 94, respectively, are provided for tubes 14 and 15, source 93 being shunted by a volume-control potential divider 95, the adjustable tap of which is coupled to the suppressor electrode of tube 14. A load circuit 98 is provided for diode rectifier 81 and the output voltage whole without regard to the details of the por- Y tion oi the system constituting the invention, per se, a desired received signal is selected and amplified in radio-frequency amplifier 10 and is converted to a modulated intermediate-frequency signal by frequency changer 13. The signal as thus converted is further amplified by intermediate-frequency amplifiers 14 and 15, and detected by detector 16, thereby producing the video-frequency modulation components which are, in turn, amplified by the video-frequency amplifier 11 and reproduced by picture reproducer 19. Diode reinserter 18 serves to insert the unidirectional components in a manner well understood in the art. It will, of course, be understood that a suitable synchronizing arrangement is provided for the receiver which may be of a conventional type and which, in the interest of simplicity, has been omitted from the circuit of Fig. 8.

Coming now to the portion of the system of Fig. 8 involving the present invention, tube 11 may have a characteristic which follows a power law, for example, it may have a characteristic in accordance with the curve of -y=2 of Fig. 4. A bias voltage for properly locating a signal-input voltage to tube 11 of a predetermined magnitude, developed by detector 16 across a resistor 80, bypassed for carrier-frequency signals by a condenser 8l, is developed across a cathode-biasing resistor 82 by-passed for carrier-frequency signals by a condenser 83. The output circuit of thereof is applied positively to the input circuit of tube 11 by means of a filter, comprising a series resistor 81 and shunt condensers 99, and the load circuit 80, 8| of detector 16.

In considering the operation of the portion of the system of Fig. 8 involving the present invention, it will be assumed that tube 11 has a powerlaw characteristic and that the video-signal output of detector 16 is applied thereto in accordance with the principles illustrated in Fig. 4 of the drawings. Thus, for example, the tube 11 may have the characteristic of 7:2 of Fig. 4. It will be asumed that the signal output of detec tor 16 is of a conventional type that is, comprises synchronizing pulses of uniform height, the signal being stabilized on the tips of the negative synchronizing pulses due 'to the fact that the unidirectional component of the signal is present in the output circuit of detector 16. For a given signal-input amplitude, therefore, the level B of the video-frequency si al corresponding to picture black may be properly related to the input voltage-output current characteristic of tube 11 by properly proportioning the cathode-biasing resistor 82. .As explained above. it is desirable that the signal level B, corresponding to picture black, as related to absolute black Bo, vary directly in accordance with the amplitude of the received signal. A proper bias for providing this result is supplied by rectifier 81 which derives a voltage which varies in accordance with the amplitude of the intermediate-frequency signal of the receiver and which is applied positively to the input circuit of tube 11. It is thus seen that the system may be so proportioned that changes in the amplitude of the intermediate-frequency signal cause the proper bias to be produced by rectifier 81. The contrast ratio of the receiver may be controlled by adjusting the variable tap on resistor 95 and it is apparent that the bias voltage developed by diode rectifier 81 is responsive either to an amplitude change of this type or to a change in the amplitude of the intermediate-frequency signal caused by a change in the amplitude of the. received signal. Thus, a proper bias is provided under the condition of operation described to procure a true and precise adjustment ofethe gamma of the signal in accordance with the power-law characteristic of the tube 11.

In summary, therefore, it will be seen that the arrangement of Fig. 8 includes a television signal-translating system comprising, the signaltranslating stage H having a signal input-signal output characteristic approximating a power-law curve, means including detector 16 for applying to the input circuit or the stage a video-irequency television signal of variable amplitude and including components representative of the unidirectional background of the translated picture, and means comprising resistor 82 for biasing stage 11 so that, for a givenamplitude of the translated signal, a predetermined signal level related to the signal, specifically, the signal level corresponding to absolute black, and outside the range oi signal-amplitude levels to be reproduced, corresponds with the intersection oi the power-law curve representing the characteristic of tube 11 and an axis oi symmetry thereof. The system also comprises means including rectiller 81 for adjusting the bias of stage 11 in accordance with the amplitudeoi' the translated signal so that predetermined shade levels of the signal are maintained at points on the powerlaw characteristic which are displaced from the axis of symmetry in accordance with their amplitude displacements from the above-mentioned 5 related signal level. 'The system also comprises load resistor 84 for deriving an output signal from the stage.

It will, of course, be understood that the source of bias voltage 81, 96 can be replaced by a manually controllable source of bias potential for tube 11. In this event, the bias would need to be manually adjusted for each change in the amplitude level 0f the intermediate-frequency signal of the receiver. Also, it will be understood that such a manually controllable source of bias for tube 11 can be unicontrolled with the volume control of the receiver, that is, can be unicongriolled with the variable tap on voltage divider While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fallwithin the true spirit and scope of the invention.

What is claimed is: a

1; The method of imparting to a television signal-translating system a translating characteristic oi'a predetermined gamma which comprises, translating with a signal input-signal output distortion corresponding to a power-law curve a television signal including components representative of the unidirectional background of the translated picture, effectively maintaining a predetermined amplitude level of the translated signal related to and outside of the amplitude range of signals to be reproduced at the intersection of said curve and an axis of symmetry thereof, whereby other predetermined amplitude levels of the translated signal corresponding to the amplitude range oi signals to be reproduced correspond to points on said curve which are displaced from said axis of symmetry in accordance with their amplitude displacement from said related level, and deriving an output signal corresponding to the distorted translated signal.

2. The method of imparting to a television signal-translating system a characteristic of a predetermined gamma which comprises, translating with a signal input-signal output distortion corresponding to a power-law curve a video-irequency television signal, eflectlvely maintaining a predetermined amplitude level or the translated signal related to and outside of the amplitude range of signals to be reproduced at the intersection of said curve and an axis of symmetry thereof, whereby other predetermined amplitude levels of the translated signal corresponding to the amplitude range of signals to be reproduced correspond to points on said curve which are displaced irom said axis of symmetry in accordance with their amplitude displacement irom rsaid related level, and deriving an output signal corresponding to the distorted translated signal.

3. The method of imparting to a television signal-translating system a translating characteristic of a predetermined gamma which comprises, translating with an input voltage-output current distortion approximately corresponding to a power-law curve a television signal including components representative of the unidirectional background of the translated picture, eii'ectively maintaining a predetermined amplitude level of the translated signal related to and outside oi the amplitude range oi signals to be reproduced at the intersection of said curve and an axis of symmetry thereof, whereby other predetermined amplitude levels 01 the translated signal corresponding to the amplitude range of signals to be reproduced correspond to points on said curve which are displaced from said axis 0! symmetry in accordance with their amplitude displacement from said related level, and deriving an output signal corresponding to the distorted translated signal.

4. The method of imparting to a television signal-translating system a translating characteristic of a predetermined gamma which comprises. translating with a signal input-signal output distortion corresponding to a power-law curve a television signal including components representa- ,tive of the unidirectional background or the translated picture, effectively maintaining a predetermined amplitude level of the translated signalcorresponding to the shade of absolute black at the intersection of said curve and an axis of symmetry thereof, whereby other predetermined amplitude levels of the translated signal corresponding to the amplitude range of signals to be reproduced correspond to points on said curve which are displaced from said axis of symmetry in accordance with their amplitude displacement from said level corresponding to absolute black, and deriving an output signal corresponding to the distorted translated signal.

5. The method of imparting to a television signal-translating system a translating characteristic of predetermined gamma which comprises,

translating with a signal input-signal output distortion corresponding to a power-law curve a television signal including components representative of the unidirectional background of the translated picture and pulses of large amplitude occurring during intervals between successive scanning lines, effectively maintaining a predetermined amplitude level or said pulses at the intersection of said curve and an axis of symmetry thereof, whereby other predetermined amplitude levels of the translated signal corresponding to the amplitude range of signals to be reproduced correspond to points on said curve which are displaced from said axis of symmetry in accordance withtheir amplitude displacement from said related level, and deriving an output signal corresponding to the distorted translated signal.

6. The method of imparting to a television signal-translating system a translating characteristic of a predetermined gamma which comprises, translating with a signal input-signal output distortion corresponding to a power-law curve a video-frequency television signal having a predetermined gamma and including components representative of the unidirectional background of the translated picture, effectively maintaining a predetermined amplitude level of the trans lated signal related to and outside of the amplitude range of signals to be reproduced at the intersection of said curve and an axis of symmetry thereof, whereby other predetermined amplitude levels of the translated signal corresponding to the amplitude range of signals to be reproduced correspond to points on said curve which are displaced from said axis of symmetry in accordance with their amplitude displacement from said related level, and deriving an output signal having a gamma modified in accordance with the exponent of said power-law curve.

7. The method of imparting to a television signal-translating system a translating characteristic of a predetermined gamma which comprises, translating with a signal input-signal output distortion corresponding to a power-law curve a television signal having the darker shade values thereof more negative than the brighter shade values thereof and including components representative of the unidirectional background of the translated picture, effectively maintaining a predetermined amplitude level of the translated signal related to and more negative than the darkest shade value at the intersection of said curve and an axis of symmetry thereof, whereby other predetermined amplitude levels of the translated signal corresponding to the amplitude range of signals to be reproduced correspond to points on said curve which are displaced from said axis of symmetry in accordance with their amplitude displacement from said related level, and deriving an output signal corresponding to the distorted translated signal.

8. The method of imparting to a television signal-translating system a translating characteristic of a predetermined gamma which com- A prises, translating with a signal input-signal output distortion corresponding to a power-law curve a television signal having darker shade values thereof less negative than the brighter shade values thereof and including components representative of the unidirectional background of the translated picture, effectively maintaining a predetermined amplitude level of the translated signal less negative than the darkest shade value at the intersection of said curve and an axis of symmetry thereof, whereby other predetermined amplitude levels of the translated signal corresponding to the ampfi' ude range of signals to be reproduced correspond to points on said curve which are displaced from said axis of symmetry in accordance with their amplitude displacement from said related level, and deriving an output signal corresponding to the distorted translated signal.

9. A television signal-translating system comprising, a signal-translating stage having a signal input-signal output characteristic approximating a power-law curve, means for applying to the input circuit of said stage a video-frequency television signal of variable amplitude and including components representative of the unidirectional background of the translated picture, means for biasing said stage so that for a given amplitude of said signal a predetermined signal level related to said signal and outside the range of signalamplitude levels to be reproduced corresponds with the intersection of said power-law curve and an axis of symmetry thereof, means for adjusting the bias of said stage in accordance with the amplitude of the translated signal so that predetermined shade levels of said signal are maintained at points on said power-law characteristic which are displaced from said axis of symmetry in accordance with their amplitude displacements from said related signal level, and means for deriving an output signal from said stage.

10. A television signal-translating system comprising, a signal-translating stage having a signal input-signal output characteristic approximating a power-law curve, means for applying to said stage a video-frequency television signal of variable amplitude and including components representative of the unidirectional background of the translated picture, means for biasing said stage so that for a given amplitude of said signal a predetermined signal level related to said signal and outside the range of signal-amplitude levels to be reproduced corresponds with the intersection of said power-law curve and an axis of symmetry thereof, means responsive to the amplitude of said signal for adjusting the bias of said stage so that predetermined shade levels of said signal are maintained at points on said power-law characteristic which are displaced from said axis of symmetry in accordance with their amplitude displacement from said related signal level, and means for deriving an output signal from said stage. I

HAROLD M. LEWIS. JOHN C. WILSON. 

